Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g. AlGaAs) have been developed since the mid-80's (K. Kinoshita et al., IEEE J. Quant. Electron. QE-23, 882 [1987]). They have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years (K. D. Choquette et al., Proc. IEEE 85, 1730 [1997]). With the success of these near-infrared lasers, attention in recent years has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (C. Wilmsen et al., Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001). There are many potential applications for visible lasers, such as, display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (T. Ishigure et al., Electron. Lett. 31, 467 [1995]). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes (either edge emitters or VCSELs) that produce light output that spans the visible spectrum.
In an effort to produce visible wavelength VCSELs, it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials can enjoy a number of advantages over inorganic-based gain materials in the visible spectrum. For example, typical organic-based gain materials have the properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers are relatively inexpensive to manufacture, can be made to emit over the entire visible range, can be scaled to arbitrary size and, most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip. Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The laser gain material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as, microcavity (Kozlov et al., U.S. Pat. No. 6,160,828), waveguide, ring microlasers, and distributed feedback (see also, for instance, G. Kranzelbinder et al., Rep. Prog. Phys. 63, 729 [2000] and M. Diaz-Garcia et al., U.S. Pat. No. 5,881,083). A problem with all of these structures is that, in order to achieve lasing, it was necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities since this generally results in more compact and easier to modulate structures.
A main barrier to achieving electrically-pumped organic lasers is the small carrier mobility of organic material, which is typically on the order of 10−5 Cm2/(V-s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (V. G. Kozlov et al., J. Appl. Phys. 84, 4096 [1998]). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales as the carrier mobility), low carrier mobilities result in orders of magnitude more charge carriers than singlet excitons; one of the consequences of this is that charge-induced (polaron) absorption can become a significant loss mechanism (N. Tessler et al., Appl. Phys. Lett. 74, 2764, [1999]). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of ˜100 W/cm2 (M. Berggren et al., Nature 389, 466, [1997]), and ignoring the above mentioned loss mechanisms, would put a lower limit on the electrically-pumped lasing threshold of 1000 A/cm2. Including these loss mechanisms would place the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, which can be supported by organic devices (N. Tessler, Adv. Mater. 19, 64 [1998]).
An alternative to electrical pumping for organic lasers is optical pumping by incoherent light sources, such as, light emitting diodes (LEDs), either inorganic (M. D. McGehee et al. Appl. Phys. Lett. 72, 1536 [1998]) or organic (Berggren et al., U.S. Pat. No. 5,881,089). This possibility is the result of unpumped organic laser systems having greatly reduced combined scattering and absorption losses (˜0.5 cm−1) at the lasing wavelength, especially when one employs a host-dopant combination as the active media. Even taking advantage of these small losses, the smallest reported optically-pumped threshold for organic lasers to date is 100 W/cm2 based on a waveguide laser design (M. Berggren et al., Nature 389, 466 [1997]). Since off-the-shelf inorganic LEDs can only provide up to ˜20 W/cm2 of power density, a different route must be taken to provide for optically pumping by incoherent sources. In order to lower the lasing threshold additionally, it is necessary to choose a laser structure which minimizes the gain volume; a VCSEL-based microcavity laser satisfies this criterion. Using VCSEL-based organic laser cavities should enable optically-pumped power density thresholds below 5 W/cm2. As a result practical organic laser devices can be driven by optically pumping them with a variety of readily available, incoherent light sources, such as LEDs.
It would be highly desirable to have an organic laser that produces linear laser light output as a function of increased power density. Unfortunately for actual organic-based VCSEL devices, as shown in FIG. 1, the laser light output varies non-linearly with pump-beam power density. The VCSEL laser cavity used to produce the data was composed of a 23 layer bottom dielectric stack of TiO2 and SiO2 (peak reflectivity of 99.3% at 560 nm), a 0.496 μm thick periodic gain (Corzine et al. IEEE J. Quant. Electr. 25, 1513 [1989]) active region, and a 29 layer top dielectric stack of TiO2 and SiO2 (peak reflectivity of 99.98% at 560 nm). The periodic gain active region contained two 0.025 μm thick layers of aluminum tris(8-hydroxyquinoline) [Alq] doped with 0.5% of [10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-[1]Benzopyrano[6,7,8-ij]quinolizin-11-one] (C545T), separated by layers of 1,1-Bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC). The pump-beam was the focused output (1000 mm lens) from a 403 mn 5 mW Nichia laser diode with a repetition rate of 5 KHz and a pulse width of 50 nsec, where its power density was varied by usage of two optical filter wheels. In addition to the saturation of the laser output power, organic VCSEL laser devices also show the undesirable property that the spectral width of the laser emission increases dramatically with pump-beam power density, as opposed to the ideal situation where the linewidth remains relatively invariant with pump-beam power density. This result is illustrated in FIG. 2, where the relative increase in the spectral width of the laser emission is plotted for the device described above. The data in FIG. 2 was obtained by reimaging the laser output (using a 50 mm collecting lens and a 100 mm focusing lens) onto the entrance slit of JY Horiba TE-cooled double monochromator (0.55 m length).